This disclosure relates to a printing apparatus and method. Specifically, the disclosed printing apparatus and method relate to scanning a P/R (Photoreceptor Belt) for image non-uniformities and controlling the printing process to reduce or correct the image non-uniformities.
A typical electrophotographic printing machine employs a photoconductive member that is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image on the photoconductive member corresponding to the image contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the photoconductive surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a sheet. Thereafter, pressure and heat are applied to the toner image to fuse the toner image to the sheet.
It is highly desirable to use an electrophotographic printing machine of this type to produce color prints. In order to produce a color print, the printing machine includes a plurality of stations. Each station has a charging device for charging the photoconductive surface, an exposing device for selectively illuminating the charged portions of the photoconductive surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a sheet. The transferred multi-color image is then permanently fused to the sheet forming the color print.
Cross-process non-uniformities, commonly referred to as streaks, can be a significant factor effecting the overall quality of a printed media sheet, for example a printed cut-sheet. Conventional printing technologies contain several sources of streaks which cannot be satisfactorily controlled via printer design or printing system optimization.
One current approach to correct for streaks is a service tool. The service tool provides correction of stable sources of spatial low-frequency non-uniformities in prints, such as the raster output system (ROS) fast-scan spot size profile. A printed image non-uniformity is scanned or sensed using an offline spectrophotometer connected to a Portable Work Station (PWS). Image corrections are controlled by a ROS intensity profile via a rolloff correction curve. This system and method of image correction is successful for some non-uniformities. However, time-varying and/or narrow streaks cannot always be controlled/corrected using this offline technique.